The present invention is generally related to interferometers. Specifically, the present invention is related to interferometers for use as spectrometers, such as Fourier transform spectrometers.
Interferometers have historically enjoyed a wide variety of applications for analyzing material properties. For example, as incorporated in a Fourier transform spectrometer, an interferometer may be used in the medical field to detect and measure various constituents of body tissues and fluids. Interferometer spectrometers are particularly useful in the medical field because they allow for relatively non-invasive measurement techniques, as compared to prior art techniques which require tissue and/or fluid sampling by physically removing the sample from the patient.
The ability to perform relatively non-invasive procedures in the measurement of body tissue and/or fluid characteristics provides a tremendous advantage over the relatively invasive procedures of the prior art. For example, U.S. Pat. No. 5,830,132 to Robinson describes a robust and accurate non-invasive analyte monitor utilizing a light dispersion device such as an interferometer spectrometer for the measurement of blood constituents including glucose, alcohol, BUN (blood urea nitrogen), bilirubin, hemoglobin, creatin, cholesterol, and electrolytes. Another example of a non-invasive analyte monitor is disclosed in U.S. Pat. No. 5,655,530 to Messerschmidt. The system and method of Messerschmidt ""530 utilizes spectrographic techniques in conjunction with an improved optical interface. As applied to the measurement of blood glucose levels, the analyte monitors disclosed in Messerschmidt ""530 and Robinson ""132 provide a diabetic patient with the opportunity to greatly improve control of the disease by more frequent or even continuous glucose monitoring, which translates into a reduction in diabetic related complications, an increase in patient comfort, an increase in life expectancy, and an overall improvement in daily life coping with the disease.
Continuous or at least more frequent glucose monitoring is achieved by eliminating the necessity to obtain a blood or other fluid sample. Practically speaking, a blood sample may not be obtained on a continuous basis nor at a sufficient frequency due to obvious reasons associated with risk of infection, patient discomfort, and patient lifestyle. The analyte monitors disclosed in Messerschmidt ""530 and Robinson ""132 overcome these obstacles by providing a non-invasive and painless means to measure blood glucose levels, thereby eliminating risk of infection and patient discomfort.
From the foregoing, it is apparent that interferometer spectrometers may have a significant impact on continuing efforts to improve the health of chronically ill patients, such as diabetics, by providing a significant improvement over prior art systems and methods of analyzing bodily tissues and/or fluids. However, this and many other applications require, for practical purposes, a relatively compact and robust interferometer. Specifically, a practical application of an interferometer spectrometer requires that the interferometer be compact such that it is portable and robust such that it is able to withstand the abuse of everyday use. Unfortunately, the prior art interferometers are, relatively speaking, neither sufficiently compact nor sufficiently robust to provide a practical interferometer spectrometer for portable use.
Probably the most famous interferometer design is the Michelson interferometer, which is commonly used for Fourier transform spectroscopy. A form of Michelson interferometer commonly used for Fourier transform spectroscopy includes six (6) basic components, namely, a collimated light source, a beam splitter, a compensator, a fixed flat end mirror, a movable flat end mirror, and a light detector. The movable end mirror may be translated along an axis perpendicular to its surface to generate a series of optical path length differences (OPD) used to measure the spectral properties of the light.
In use, light emitted from the light source strikes the beam splitter, which partially reflects and partially transmits the light therethrough. The reflected beam travels to the movable mirror and is reflected back through the beam splitter toward the detector. The transmitted beam travels through the compensator plate (same thickness and material as the beam splitter plate) to the fixed end mirror and is reflected back through the compensator plate, reflected off of the beam splitter and toward the light detector.
As mentioned previously, the movable mirror may be translated back and forth with a finely calibrated screw adjustment, or the like, to generate an optical path length difference (OPD) or cause retardation such that the recombined beam forms an interference pattern, commonly referred to as an interferogram. Retardation is the OPD between a pair of output rays originating from a single input ray. By observing the interference pattern, and measuring the distance the movable mirror is translated, the wavelength of the light provided by the light source may be determined. Further, changes in wavelength may be measured to determine the index of refraction of test samples which may then be used to identify the material and characteristics of the test sample. Further yet, by observing the interference pattern at various wavelengths, the amount of light absorbed by test sample may be measured, which is indicative of the material and properties of the test sample.
Although the Michelson interferometer is extremely useful, it tends to be relatively sensitive to alignment of its various components. In particular, a tilt error is created by a change in the angle of the beam splitter, the fixed-end mirror, or the movable-end mirror relative to the other components. Tilt error may be defined as a deviation from strict parallelism of a pair of output rays originating from a single input ray. The effect of a tilt error is to reduce the modulation efficiency of the interferometer, in a wavelength dependent manner, causing a spectral calibration error. For example, a change in angle of an end mirror, corresponding to an edge displacement (relative movement of opposite edges of the end mirror) by less than five percent (5%) of the wavelength of the light, causes an unacceptable change in calibration of the interferometer. This type of alignment sensitivity is particularly difficult to eliminate with regard to the movable end mirror.
Attempts have been made, with limited success, to eliminate the tilt error of the Michelson interferometer by replacing the flat end mirror with retroreflectors as described by W. H. Steel, xe2x80x9cInterferometers for Fourier Spectroscopy,xe2x80x9d Aspen International Conference on Fourier Spectroscopy, pp. 43-53 (1970). Although replacing the flat end mirrors with retroreflectors, such as cube-corner type or xe2x80x9ccat""s-eyexe2x80x9d type retroreflectors, eliminate tilt error, a shear error may be caused by the lateral displacement of either retroreflector or a tilt of the beam splitter. Shear error is the lateral displacement of one light path relative to the other light path which causes a wavelength dependent reduction in the modulation efficiency of the interferometer. Shear error may be defined as a lateral separation of a pair of parallel output rays originating from a single input ray when the optical path difference (OPD) between the two rays is zero. Even a relatively small shear error on the order of a few wavelengths of light may be detrimental to the calibration of the interferometer.
Other attempts have been made to improve on the Michelson interferometer design in an effort to reduce alignment sensitivity of the components. For example, the Folded Jamin design provides a relatively stable design utilizing a relatively thick beam splitter plate and a rocking mirror as described by L. Mertz, xe2x80x9cTransformations in Optics,xe2x80x9d page 50 (1965). Although the Folded Jamin design reduces component alignment sensitivity, an exact ray trace analysis of the design demonstrates that the allowable field of view (FOV) is relatively small, particularly as compared to the FOV of the Michelson interferometer. A relatively small FOV renders the Jamin interferometer unsuitable for Fourier transform spectroscopy, particularly when the signal-to-noise ratio must be optimized through the use of a light source of a large angular subtense.
Further attempts have been made to reduce the alignment sensitivity of the Michelson interferometer by rotating the interferometer components as a group to generate the OPD. For example, U.S. Pat. No. 4,684,255 to Ford and the article by R. S. Sternberg and J. F. James xe2x80x9cA New Type Of Michelson Interference Spectrometer,xe2x80x9d J. Sci. Instru., Vol. 41 (1964) pp. 225-226, describe interferometers wherein the OPD is generated by rotating four components as a group. Another example is disclosed in U.S. Pat. No. 5,537,208 to Bertram et al. which describes an interferometer wherein the OPD is generated by rotating two mirrors in parallel. Although tilt error and shear error are eliminated by these designs to the extent that the components are rotated as a group with no relative movement therebetween, tilt and shear error may be caused by an incorrectly positioned component as constructed. As such, these designs inherently rely on the precise positioning and mounting of the components, as constructed and maintained thereafter, to eliminate tilt and shear error. For example, European Patent Application 0681166 A1 proposes the use of optically flat and parallel spacers to establish optical contact between the critical components and thereby maintain the precise position of the components. However, such component mounting techniques are relatively costly to implement.
In sum, many of the interferometer spectrometers proposed in the prior art are sensitive to relative alignment between the critical components, and thus are susceptible to tilt error and/or shear error. Attempts to reduce the alignment sensitivity of the various components have been met with limited success. Specifically, interferometer spectrometers of the prior art that reduce tilt and/or shear error have done so by compromising other performance aspects of the design and by increasing manufacturing costs.
The interferometer spectrometer of the present invention reduces alignment sensitivity of the critical components without compromising performance or increasing manufacturing costs. Specifically, as compared to the Michelson interferometer, the interferometer of the present invention does not produce tilt error due to relative tilting of the components. As compared to the modified Michelson interferometer utilizing retroreflectors, the interferometer spectrometer of the present invention greatly reduces shear error due to tilting or lateral movement of any of the components. In addition, as compared to the Jamin interferometer, the interferometer spectrometer of the present invention provides a much larger FOV. Further, as compared to the component group rotation interferometers, the interferometer spectrometer of the present invention eliminates tilt and shear sensitivity of the individual components, as opposed to groups of components, thereby providing a more stable design with less complex and lower-cost component mounting techniques. Further yet, as compared to prior art interferometer spectrometers that are field-widened, the interferometer spectrometer of the present invention is field-widened without introducing the possibility of tilt and/or shear error.
The present invention overcomes the disadvantages of the prior art by providing an interferometer spectrometer that has reduced alignment sensitivity. In particular, variations in relative alignment (angular or translational displacement) do not adversely affect the parallelism (i.e., tilt error) of the recombined output ray pair, and thus do not result in calibration error. In addition, translational variations in relative alignment do not change the separation (i.e., shear error) of the output ray pair, and thus do not result in calibration error. Furthermore, rotational variations in relative alignment produce very little separation (i.e., shear error) of the output ray pair, and thus reduce sensitivity to mounting alignment and stability tolerances as compared to a Michelson interferometer with cube-comer or xe2x80x9ccat""s-eyexe2x80x9d retroreflectors. The reduced alignment sensitivity may be accomplished by utilizing simple planar components that are common to both light paths. The reduced alignment sensitivity and simplicity in design provides a more compact and more robust interferometer, with reduced manufacturing costs associated therewith.
In an exemplary embodiment of the present invention, the interferometer spectrometer includes a beam splitter, a means for redirecting the split back toward the beam splitter, and a means for generating a path length difference (OPD) between the split rays. Both of the split rays optically interact with each of the beam splitter, the redirecting means, and the means for generating a path length difference, thereby reducing alignment sensitivity. The split rays are recombined by the beam splitter to form an output ray pair, wherein the rays forming the output ray pair are parallel. The interferometer may include a compensator, and the path length difference generating means may comprise rotation of the beam splitter, the redirecting means, or the compensator.
With this arrangement, translational and rotational changes in relative position between the beam splitter, the redirecting means, and the means for generating a path length difference do not result in a lack of parallelism between the rays forming the output ray pair. Further, translational changes in relative position between the beam splitter, the redirecting means, and the means for generating a path length difference do not result in a lateral separation of the rays forming the output ray pair. Further yet, there is no lateral separation of the rays forming the output ray pair when the first and second rays strike the end mirror at normal incidence.
In another exemplary embodiment of the present invention, the interferometer spectrometer includes a beam splitter, an end mirror and a means for generating an optical path length difference (OPD). The interferometer may also include a compensator and a scanner plate disposed between the beam splitter and the end mirror. The beam splitter causes an input ray to be split into a first ray and a second ray having a first path and a second path, respectively. The end mirror terminates the first and second paths to define a first path length and a second path length, respectively. The end mirror also reflects the first and second rays back to the beam splitter to combine the rays into an output ray pair. The OPD generating means causes a difference between the first and second path lengths to create varying amounts of constructive or destructive interference between the two output rays. The OPD may be generated by rotating the beam splitter, the compensator, or the scanner plate. The output ray pair has a substantial degree of parallelism, which is independent of variations in the relative translational or angular position of the components and a separation which is independent of variations in the relative translational position of the components. Preferably, both the first and second rays are common to the beam splitter and the compensator, and both rays reflect off one end mirror. The beam splitter and the compensator each preferably have a simple planar geometry such that the first and second rays are parallel to each other after passing therethrough.
In yet another exemplary embodiment of the present invention, the light source for an interferometer spectrometer produces an elliptical angular subtense. The elliptical angular subtense light source of the present invention provides an interferometer spectrometer having an increased throughput relative to an interferometer utilizing a light source of circular angular subtense. The light source may include a single collimator lens or an array of collimators lenses each having an array of transmitting fibers disposed adjacent an array of receiving fibers. The array of collimator lenses provides a more compact design than a single circular collimator lens of the same area, and is suitable for single channel or multi-channel use.